The resin transfer molding (RTM) process has become a popular composite manufacturing process due to its suitability for high volume production and cost effectiveness [see references 1, 2]. In this process, the dry fiber reinforcement (preform) is enclosed in the mold and resin is injected and allowed to cure. Resin injection or transferability is defined by the permeability of the preform and it can be severely affected by defects, distortions or anomalies in the preform. In some cases, such as those involving complex geometries, it is not uncommon for the preform to be misplaced or shift and be distorted during mold closure. This fact, if not detected and corrected before resin injection, results in costly scrapping of the finished part [see reference 3]. Preform defects can cause local permeability non-uniformities and affect the resin flow resulting in local resin-starved areas. Furthermore, preform distortions could contribute to residual stresses and undesirable stress concentrations during subsequent loading in service. Thus, detection of preform defects and irregularities is a very important quality control step for reducing costly scrapping and insuring the quality and reliability of fabricated composite structures.
The impregnation of the fluid into the preform is defined by the permeability of the reinforcement, which is the ability of a Newtonian fluid to permeate a porous medium with a sufficiently low Reynolds number, as given by Darcy's Law. The permeability of an undistorted preform with a constant fiber volume ratio can be assumed to be uniform over the entire domain; however, the permeability can be significantly altered by defects, distortions, or other anomalies in the preform. Such drastic changes in local permeability can affect resin flow patterns, rendering portions of the mold to be insufficiently filled. Permeability variations within a preform can be attributed to a number of factors, such as improper preform preparation, misplacement or shifting in the mold, accidental inclusion of foreign material, natural surface density variation of the preform, etc. If such occurrences were not detected prior to resin injection, the potential for costly part scrapping would be increased. Aside from the additional voids due to permeability non-uniformities, preform distortions could contribute to residual stresses and stress concentrations during in-service loading. Early detection, therefore, of such reinforcement irregularities is critical for effective quality control.
Conventional methods for permeability measurement rely on oils or other viscous fluids to be injected into the reinforcement. These fluids soil the fibers and cannot be removed without damage to the preform. Furthermore, the bulk material permeability data obtained from such tests would not provide the information regarding local permeability variation, which is necessary for locating defects. Such strategies, therefore, could not logically be applied in-situ prior to an RTM run. Methods, however, providing multiple pressure measurements with a gas, like the gas flow method, are well suited for such applications.
Published US Patent Application US 2002/0046596 A1 describes a system for in-situ and on-line monitoring of a preform layup process for liquid composite molding using flow of a compressed gas through the preform in the mold cavity. Pressure transducers communicated to openings in a lower mold section provided sensed gas pressure values to a data acquisition and processing device having software which generates a pressure profile of the gas flow for the preform. This pressure profile is evaluated by comparing it with a theoretically calculated pressure profile. The method described in the patent application is inherently one-dimensional and was not proven with preform materials having varying degrees of anisotropy. Furthermore, it requires prior determination of the permeability of the perform material.